Specific recognition of foreign antigens by T lymphocytes triggers a biochemical signal that leads to activation of gene transcription and effector machinery including the cytotoxic functions and elaboration of cytokines. Among the cytokines released is IL-2 which serves as an autocrine growth factor to drive T cell proliferation. The earliest identifiable biochemical event in response to antigen and IL2 stimulation of T cells is tyrosine phosphorylation of cellular proteins including the receptor components (e.g. TCR zeta chain, ZAP-70, and PLCgamma1), and inhibition of tyrosine phosphorylation completely abrogates downstream events in T cell activation. Members of the Src family of tyrosine kinases, p59tyn (Fyn) and p56lck (Lck) physically interact with the cytoplasmic signalling domains of TCRzeta/CD3 chains and CD4/8 respectively. Lck also physically interacts with the IL2R beta chain. Fyn, Lck and other Src-family tyrosine kinases also participate in signalling through other leukocyte receptors, such as B-cell receptor (membrane Ig), CD16 on NK cells and CSF receptor on monocytes. A large number of studies including direct gene targeting have demonstrated critical roles of Fyn and Lck in TCR signalling [1-3]. Considering the pivotal role of protein tyrosyl phosphorylation in signalling through a number of leukocyte receptors, it is thought that abnormal phosphorylation amy contribute to inappropriate leukocyte activation and/or proliferation in autoimmune diseases. Consistent with this possibility, autoimmune disease-prone lpr mice show increased tyrosyl phosphorylation of proteins in T cells and a remarkable over-expression of Fyn protein [4,5].